This invention relates to a two-stage anodization process by which foil that has been etched at up to 150 V for low-voltage use (for capacitors rated to 100 V) is made suitable for intermediate to high-voltage use.
Foil that has been etched for low-voltage use typically has a fine etch structure. When the etching is electrochemical using direct current, the etch structure consists of narrow tubes or tunnels which change direction frequently and is quite filamentary. For low-voltage capacitor applications (0-100 V) the barrier-layer anodized film that is formed is relatively thin and covers the interior of these tunnels without filling or plugging them with anodic oxide, so that the surface area remains high and the fill or working capacitor electrolyte can penetrate into these tunnels.
Foil that has been electrochemically etched using direct current for high-voltage use (above 250 V) typically has a rather coarse tunnel etch structure. The tunnels are wider and not as randomly arranged, and the etch structure has been described as resembling a child's jungle gym. The wider tunnels can accommodate the thicker anodic oxide films needed for high voltage operations.
Foil that has been etched using alternating current has a very different etch structure. Pits rather than tunnels are formed, and the etch structure resembles bunches of grapes or cauliflower flowerets. A low-voltage AC etch produces pits which are very fine and can be plugged easily by anodic oxide, thereby reducing surface area and hence capacitance. A high-voltage AC etch produces deeper more rounded pits which can accommodate a thicker anodic oxide layer without drastically reducing the enhanced surface area of the foil produced by the etch.
Foil for intermediate voltage use (150 to 250 V) has a structure intermediate those described above, as is shown by both etch structures becoming more symmetrical and wider.
Thus, capacitor manufacturers must inventory a variety of etched foils in order to be able to produce capacitors throughout the various voltage ranges. To maintain a large inventory of each range of etched foils is expensive; not to do so runs the risk of running short of suitable foil in the case of a large demand in a particular voltage range.